1.1. Requirements Notation and Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3.1. JOSE and COSE secp256k1 Curve Key Representations

The SECG elliptic curve secp256k1 [SEC2] is represented in a JSON Web Key (JWK) [RFC7517] using these values:¶

• kty: EC¶
• crv: secp256k1¶

plus the values needed to represent the curve point, as defined in Section 6.2.1 of [RFC7518]. As a compressed point encoding representation is not defined for JWK elliptic curve points, the uncompressed point encoding defined there MUST be used. The x and y values represented MUST both be exactly 256 bits, with any leading zeros preserved. Other optional values such as alg MAY also be present.¶

It is represented in a COSE_Key [RFC8152] using these values:¶

• kty (1): EC2 (2)¶
• crv (-1): secp256k1 (8)¶

plus the values needed to represent the curve point, as defined in Section 13.1.1 of [RFC8152]. Either the uncompressed or compressed point encoding representations defined there can be used. The x value represented MUST be exactly 256 bits, with any leading zeros preserved. If the uncompressed representation is used, the y value represented MUST likewise be exactly 256 bits, with any leading zeros preserved; if the compressed representation is used, the y value is a boolean value, as specified in Section 13.1.1 of [RFC8152]. Other optional values such as alg (3) MAY also be present.¶

3.2. ECDSA Signature with secp256k1 Curve

The ECDSA signature algorithm is defined in [DSS]. This specification defines the ES256K algorithm identifier, which is used to specify the use of ECDSA with the secp256k1 curve and the SHA-256 [DSS] cryptographic hash function. Implementations need to check that the key type is EC for JOSE or EC2 (2) for COSE and that the curve of the key is secp256k1 when creating or verifying a signature.¶

The ECDSA secp256k1 SHA-256 digital signature is generated as follows:¶

1. Generate a digital signature of the JWS Signing Input or the COSE Sig_structure using ECDSA secp256k1 SHA-256 with the desired private key. The output will be the pair (R, S), where R and S are 256-bit unsigned integers.¶
2. Turn R and S into octet sequences in big-endian order, with each array being 32 octets long. The octet sequence representations MUST NOT be shortened to omit any leading zero octets contained in the values.¶
3. Concatenate the two octet sequences in the order R and then S. (Note that many ECDSA implementations will directly produce this concatenation as their output.)¶
4. The resulting 64-octet sequence is the JWS Signature or COSE signature value.¶

Implementations SHOULD use a deterministic algorithm to generate the ECDSA nonce, k, such as the algorithm defined in [RFC6979]. However, in situations where devices are vulnerable to physical attacks, deterministic ECDSA has been shown to be susceptible to fault injection attacks [KUDELSKI17] [EURO-SP18]. Where this is a possibility, implementations SHOULD implement appropriate countermeasures. Where there are specific certification requirements (such as FIPS approval), implementors should check whether deterministic ECDSA is an approved nonce generation method.¶

The ECDSA secp256k1 SHA-256 algorithm specified in this document uses these identifiers:¶

When using a JWK or COSE_Key for this algorithm, the following checks are made:¶

• The kty field MUST be present, and it MUST be EC for JOSE or EC2 for COSE.¶
• The crv field MUST be present, and it MUST represent the secp256k1 elliptic curve.¶
• If the alg field is present, it MUST represent the ES256K algorithm.¶
• If the key_ops field is present, it MUST include sign when creating an ECDSA signature.¶
• If the key_ops field is present, it MUST include verify when verifying an ECDSA signature.¶
• If the JWK use field is present, its value MUST be sig.¶

3.3. Other Uses of the secp256k1 Elliptic Curve

This specification defines how to use the secp256k1 curve for ECDSA signatures for both JOSE and COSE implementations. While in theory the curve could also be used for ECDH-ES key agreement, it is beyond the scope of this specification to state whether this is or is not advisable. Thus, whether or not to recommend its use with ECDH-ES is left for experts to decide in future specifications.¶

When used for ECDSA, the secp256k1 curve MUST be used only with the ES256K algorithm identifier and not any others, including not with the COSE ES256 identifier. Note that the ES256K algorithm identifier needed to be introduced for JOSE to sign with the secp256k1 curve because the JOSE ES256 algorithm is defined to be used only with the P-256 curve. The COSE treatment of how to sign with secp256k1 is intentionally parallel to that for JOSE, where the secp256k1 curve MUST be used with the ES256K algorithm identifier.¶

4.1. COSE Algorithms Registrations

IANA has registered the following values in the "COSE Algorithms" registry [IANA.COSE.Algorithms].¶

Name:

RS256¶

Value:

-257¶

Description:

RSASSA-PKCS1-v1_5 using SHA-256¶

Change Controller:

IESG¶

Reference:

Section 2 of RFC 8812¶

Recommended:

No¶

Name:

RS384¶

Value:

-258¶

Description:

RSASSA-PKCS1-v1_5 using SHA-384¶

Change Controller:

IESG¶

Reference:

Section 2 of RFC 8812¶

Recommended:

No¶

Name:

RS512¶

Value:

-259¶

Description:

RSASSA-PKCS1-v1_5 using SHA-512¶

Change Controller:

IESG¶

Reference:

Section 2 of RFC 8812¶

Recommended:

No¶

Name:

RS1¶

Value:

-65535¶

Description:

RSASSA-PKCS1-v1_5 using SHA-1¶

Change Controller:

IESG¶

Reference:

Section 2 of RFC 8812¶

Recommended:

Deprecated¶

Name:

ES256K¶

Value:

-47¶

Description:

ECDSA using secp256k1 curve and SHA-256¶

Change Controller:

IESG¶

Reference:

Section 3.2 of RFC 8812¶

Recommended:

No¶

4.2. COSE Elliptic Curves Registrations

IANA has registered the following value in the "COSE Elliptic Curves" registry [IANA.COSE.Curves].¶

Name:

secp256k1¶

Value:

8¶

Key Type:

EC2¶

Description:

SECG secp256k1 curve¶

Change Controller:

IESG¶

Reference:

Section 3.1 of RFC 8812¶

Recommended:

No¶

4.3. JOSE Algorithms Registrations

IANA has registered the following value in the "JSON Web Signature and Encryption Algorithms" registry [IANA.JOSE.Algorithms].¶

Algorithm Name:

ES256K¶

Algorithm Description:

ECDSA using secp256k1 curve and SHA-256¶

Algorithm Usage Location(s):

alg¶

JOSE Implementation Requirements:

Optional¶

Change Controller:

IESG¶

Reference:

Section 3.2 of RFC 8812¶

Algorithm Analysis Document(s):

[SEC2]¶

4.4. JSON Web Key Elliptic Curves Registrations

IANA has registered the following value in the "JSON Web Key Elliptic Curve" registry [IANA.JOSE.Curves].¶

Curve Name:

secp256k1¶

Curve Description:

SECG secp256k1 curve¶

JOSE Implementation Requirements:

Optional¶

Change Controller:

IESG¶

Specification Document(s):

Section 3.1 of RFC 8812¶

5.1. RSA Key Size Security Considerations

The security considerations on key sizes for RSA algorithms from Section 6.1 of [RFC8230] also apply to the RSA algorithms in this specification.¶

5.2. RSASSA-PKCS1-v1_5 with SHA-2 Security Considerations

The security considerations on the use of RSASSA-PKCS1-v1_5 with SHA-2 hash functions (SHA-256, SHA-384, and SHA-512) from Section 8.3 of [RFC7518] also apply to their use in this specification. For that reason, these algorithms are registered as being "Not Recommended". Likewise, the exponent restrictions described in Section 8.3 of [RFC7518] also apply.¶

5.3. RSASSA-PKCS1-v1_5 with SHA-1 Security Considerations

The security considerations on the use of the SHA-1 hash function from [RFC6194] apply in this specification. For that reason, the "RS1" algorithm is registered as "Deprecated". Likewise, the exponent restrictions described in Section 8.3 of [RFC7518] also apply.¶

A COSE algorithm identifier for this algorithm is nonetheless being registered because deployed Trusted Platform Modules (TPMs) continue to use it; therefore, WebAuthn implementations need a COSE algorithm identifier for "RS1" when TPM attestations using this algorithm are being represented. New COSE applications and protocols MUST NOT use this algorithm.¶

5.4. secp256k1 Security Considerations

Care should be taken that a secp256k1 key is not mistaken for a P-256 [RFC7518] key, given that their representations are the same except for the crv value. As described in Section 8.1.1 of [RFC8152], we currently do not have any way to deal with this attack except to restrict the set of curves that can be used.¶

The procedures and security considerations described in the [SEC1], [SEC2], and [DSS] specifications apply to implementations of this specification.¶

Timing side-channel attacks are possible if the implementation of scalar multiplication over the curve does not execute in constant time.¶

There are theoretical weaknesses with this curve that could result in future attacks. While these potential weaknesses are not unique to this curve, they are the reason that this curve is registered as "Recommended: No".¶